Many buildings today utilize VAV systems to supply conditioned air to zones within the building. Normally, each will have numerous supply fans, with each fan associated with a duct system generally consisting of a main air supply duct that branches into several terminal supply ducts, with a terminal box at the terminus of each terminal supply duct. When the fans are being operated, conditioned air is continually moved through the duct system and into the zones within the building.
Conventional VAV systems are typically designed to maintain a static pressure setpoint at a single and fixed location within the duct system. The static pressure setpoint is maintained so as to provide sufficient air pressure to all of the terminal boxes based on calculated or simulated maximum design-load conditions, meaning the most thermally extreme conditions to be expected by the building. Typically, VAV system designers size terminal boxes such that the boxes are capable of supplying sufficient volumes of air to each of the zones within the building in order to satisfy maximum design-load conditions. The designers then establish the static pressure setpoint at a location that is approximately two-thirds of the distance from the supply fan to the end of the main air supply duct. A pressure sensor is provided at that location and static pressure setpoint is measured at that location, and an air supply fan is operated at a speed that is sufficient to maintain the static pressure set point at that location. Under partial-load conditions when building is experiencing less than maximum load conditions, it is becomes necessary to reduce the volumes of air being delivered to the zones within the building by partially closing the terminal box dampers. Unfortunately, by using this method the energy efficiency of the supply fan is greatly reduced due to the friction losses due to partially closed terminal box dampers and within the duct system. Since it common for VAV systems to operate under partial load conditions during the vast majority of hours of operation, such systems are normally operating at inefficient levels.
Several different control designs have been introduced in an attempt to overcome this limitation in conventional VAV systems. Recent approaches have been to design a system that dynamically resets the static pressure setpoint based upon changing demands for conditioned air. In particular, a VAV system has been proposed in U.S. Pat. No. 5,863,246 issued to Bujak in which a system controller is utilized to reset the design static pressure setpoint, at a location down-stream from the supply fan and within the main supply duct, based upon the position of all terminal dampers within the system as compared to certain predetermined damper positions. For example, if all terminal damper positions are determined to be open less than a predetermined minimum amount, then an offset calculation is performed which increases the value of a previously calculated offset by a predetermined incremental amount. On the other hand, if any damper position is determined to be open more than a predetermined maximum amount, then a different offset calculation is performed which decreases the value of the a previously calculated offset by a predetermined incremental amount, which may differ from the previously calculated incremental amount. The system controller then adjusts the design static pressure setpoint based upon the computed offset value, thereby generating a controlling static pressure setpoint. Next, the controller compares the controlling static pressure setpoint to a sensed static pressure within the main supply duct at the setpoint location. Lastly, the pressure in the main duct at the setpoint location is adjusted by modulating the supply fan speed based upon a comparison of the sensed static pressure to the controlling static pressure.
Although Bujak improves upon some of the energy inefficiencies of conventional VAV systems, the patent discloses several of its own limitations. One of the most significant limitations is that even though the VAV system disclosed in Bujak uses an algorithm to ostensibly identify the zone of greatest demand, the algorithm always measures a static pressure at a single and fixed location in the main supply duct down-stream from the supply fan. As a result, the system in Bujak attempts to modulate the supply of conditioned air to the zone of greatest demand by continuing to incorporate the limitation, as in other VAV systems, of using a single static pressure setpoint. This approach wastes energy since it will inevitably supply more static pressure than necessary to the terminal box serving the zone that the system identifies as having the greatest demand. Another important limitation of the system disclosed in Bujak is that it introduces an inherent friction loss and an associated loss of energy efficiency because the system prevents any terminal damper from operating in a wide-open condition. Bujak discloses another limitation arising from the fact that the system introduces a large “dead band” in which no changes to the static pressure setpoint can occur, making the design unresponsive to changes in demand for conditioned air within the zones within the building.
As will be discussed below, the VAV system of the present invention substantially improves upon the limitations in traditional VAV systems.